The present invention relates generally to the field of electrophysiology. More particularly, this invention relates to methods and apparatus for treating cardiac arrhythmias.
Symptoms of abnormal heart rhythm are generally referred to as cardiac arrhythmias, with an abnormally slow rhythm being classified as a bradycardia and an abnormally rapid rhythm being referred to as a tachycardia. The present invention is concerned with the treatment of tachycardias which are frequently caused by the presence of an "arrhythmogenic site" or "accessory atrioventricular pathway" close to the inner surface of one of the chambers of the heart. The heart includes a number of normal pathways which are responsible for the propagation of signals necessary for normal electrical mechanical function. The presence of arrhythmogenic sites or accessory pathways can bypass or short circuit the normal pathways, potentially resulting in very rapid heart contractions, referred to as tachycardias. Tachycardias may be defined as ventricular tachycardias (VT's) and supraventricular tachycardias (SVT's). VT's originate in the left or right ventricle and are typically caused by arrhythmogenic sites associated with a prior myocardial infarction. SVT's originate in the atria and are typically caused by an accessory pathway.
Treatment of both ventricular and supraventricular tachycardias may be accomplished by a variety of approaches, including drugs, surgery, implantable pacemakers/defibrillators, and catheter ablation. While drugs may be the treatment of choice for many patients, they only mask the symptoms and do not cure the underlying cause. Implantable devices only correct the arrhythmia after it occurs. Surgical and catheter-based treatments, in contrast, will actually cure the problem, usually by ablating the abnormal arrhythmogenic tissue or accessory pathway responsible for the tachycardia. The catheter-based treatments rely on the application of various destructive energy sources to the target tissue, including direct current electrical energy, radiofrequency electrical energy, laser energy, and the like.
Of particular interest to the present invention are radiofrequency (RF) ablation protocols which have proven to be highly effective in tachycardia treatment while exposing the patient to minimum side effects and risks. Radiofrequency catheter ablation is generally performed after an initial mapping procedure where the locations of the arrhythmogenic sites and accessory pathways are determined. After mapping, a catheter having a suitable electrode is introduced to the appropriate heart chamber and manipulated so that the electrode lies proximate the target tissue. Radiofrequency energy is then applied through the electrode to the cardiac tissue in order to ablate a region of the tissue which forms part of arrhythmogenic site or the accessory pathway. By successfully destroying that tissue, the abnormal signalling patterns responsible for the tachycardia cannot be sustained. A method and system for performing RF ablation by controlling temperature at the ablation site is described in co-pending application Ser. No. 866,683, entitled "Method and System for Radiofrequency Ablation of Cardiac Tissue," the full disclosure of which is hereby incorporated herein by reference.
Catheters utilized in radiofrequency ablation are inserted into a major vein or artery, usually in the neck or groin area, and guided into the chambers of the heart by appropriate manipulation through the vein or artery. The tip of the catheter must be manipulable by the user from the proximal end of the catheter, so that the distal electrode can be positioned against the tissue region to be ablated. The catheter must have a great deal of flexibility in order to follow the pathway of the major blood vessels into the heart, and the catheter must permit user manipulation of the tip even when the catheter is in a curved and twisted configuration. Because of the high degree of precision required for proper positioning of the tip electrode, the catheter must be manipulable with a high degree of sensitivity and controllability. In addition, the distal portion of the catheter must be sufficiently resilient in order to be positioned against the wall of the ventricle and maintained in a position during ablation without being displaced by the movement of the beating heart. Along with the steerability, flexibility and resiliency, the catheter must have a sufficient degree of torsional stiffness to permit user manipulation from the proximal end.
Steerable catheters are known for use in a variety of medical procedures. See, for example, U.S. Pat. No. 4,998,916 to Hammerslag, U.S. Pat. No. 4,944,727 to McCoy, U.S. Pat. No. 4,838,859 to Strassmann, U.S. Pat. No. 4,826,087 to Chinery, U.S. Pat. No. 4,753,223 to Bremer, U.S. Pat. No. 4,685,457 to Donenfeld, U.S. Pat. No. 3,605,725 to Bentov, U.S. Pat. No. 3,470,876 to Barchilon and U.S. Pat. No. 4,960,134 to Webster, Jr. Typically, such catheters employ a plurality of steering wires, usually three or four, extending from a steering mechanism at the proximal end of the catheter to an anchor point at the distal end of the catheter. By tensioning certain of the steering wires using the control mechanism, the tip of the catheter can be manipulated in a desired direction. See, e.g., U.S. Pat. 3,470,876 to Barchilon or U.S. Pat. No. 3,605,725 to Bentov. In addition to being steerable in the lateral direction, further positioning of known catheters is accomplished by rotating the catheter as a whole about its longitudinal axis, typically by turning or twisting the proximal end of the catheter. This exerts a torque along the length of the catheter which is translated into a rotational motion at the distal end, allowing a laterally deflected distal tip to be rotated.
While radiofrequency ablation using existing catheters has had promising results, such catheters suffer from certain disadvantages. In particular, known catheters lack a sufficient degree of steering sensitivity to accurately position the distal tip electrode at the desired position within the heart. The known technique of achieving rotation of the distal tip by twisting the proximal end of the catheter frequently results in "whipping" of the distal tip in an abrupt and uncontrolled recoiling motion. Such recoiling can occur at unpredictable locations and timings, reducing the success of ablation procedures and potentially risking the health of the patient. Known catheters further suffer from the inability to maintain the position of the distal electrode against the wall of the ventricle the movement of the beating heart. In addition, known catheters suffer from a tradeoff between catheter length and torsional stiffness, with catheters of relatively long length having insufficient torsional stiffness to allow effective positioning by rotating the proximal end of the catheter.
For these and other reasons, it would be desirable to provide a catheter suitable for radiofrequency ablation which is steerable from the proximal end with greater sensitivity and controllability than in known catheters. Further, the catheter should have greater torsional stiffness per unit length than known catheters, so that the relatively long catheters required in certain ablation procedures are sufficiently manipulable from the proximal end. At the same time, such catheters should allow both lateral and rotational manipulation of the distal tip for both "rough" and "fine" positioning. Further, the catheter should have sufficient flexibility to follow the contours of the vessel pathway into the heart without detracting from the catheter's steerability.